Phase modulation



ATTORNEY 2 2 3/ 3/ f4w/ |l E Mg C Rm w wa f ow 4 AM. T D .r/ i/ 0 L M 0fw/ 40 3 No 3 /MMJ C E 6 Ma# veg W HWOM mE www in H Mul/ M46 .l 0 M4:/47 M /A|2.||v B v Y .w 1- 1: Z

March 22, 1938. H. E. GoLDsTlNE PHASE MODULAT I ON Original Filed Feb.8, 1934 March A22, 193s.

H. E. GoLDsTlNE 2,1 1 1,587

PHASE MODULATION Original Filed Feb. 8, 19311 3 sheets-shea 2 INVENTORH.E.GOLD5T|NE ATTORNEY lill-T- March 22, 1938.

H. E. GO'LDSTINE PHASE MOD ULAT ION "3 Sheets-Sheet 5 Original FiledFeb. 8, 1934 1000mm/6 l faff/vr/Azs lNvENToR H. E.GOLD5T|NE l w@ N/ wwwATTORNEY Patented Mar. 22, 1938 UNITED STATES PATENT CFFICE PHASEMODULATION Application February 8, 1934, Serial No. 710,239 RenewedOctober 27, 1936 18 Claims.

This invention relates to the art of signalling by means of highfrequency oscillations, the character of which varies in accordance withsignals, and more in particular with a method of and means fordisplacing the phase of high frequency oscillations in a correctrelationship to an applied signal voltage, thereby producing a signal somodulated that intelligent transmission may be made by using phasemodulation.

In the usual method of modulating the output current of a transmitterthe amplitude of the oscillations is made to vary in accordance with thesignal to be transmitted. When the input signal is of low amplitude, sothat a low percentage of modulation of the carriers is accomplished, theoutput of the transmitter is less than when the signal is of highamplitude and a larger percentage of modulation is impressed on thecarrier. With amplitude modulation the transmitter must be operatedbelow full power output to be able to follow the signal modulation toits peak value. When using amplitude modulation the power output of thetransmitter varies in accordance with the percent modulation. In orderthat the transmitter may be capable of carrying full output on highpercent of modulation it must be runat a value below its full outputrating on the average modulation. For example, when a broadcast programis sent out over the usual broadcast transmitter the average modulationis about 30%. The transmitter must be so adjusted that while it onlydelivers a small amount of its power on the average modulation it cancarry the peaks or the louder parts of the signal without distortion. iThus the transmitter must be operated below its full output most of thetime and less power radiated than would be possible with thistransmitter. In phase modulation, however, the phase relation of thecarrier and the side bands varies in such a manner that the resultantoutput remains constant. In phase modulation the output of thetransmitter remains constant and the modulation side bands and carriervary in such a manner that the output energy remains constant. A phasemodulated transmitter may be operated at maximum output constantly. Thisresults in increased efficiency and permits more radiation power whenusing equipment of the same power rating. The oscillations modulated inphase as indicated above may be altered in characteristics at signalfrequency in any of the succeeding stages prior to transmission. Theamplitude of the phase modulated wave may be increased or the frequencyof the phase modu- 55 lated Wave may be increased with or without infificreasing the phase shift. Furthermore, the wave may be limited as toamplitude to remove amplitude modulation components before or after theoperations mentioned in the preceding sentence.

' In the usual type of phase modulation trans- 5 mitters, elaborateequipment is needed to produce quality of modulation.

An object of the present invention is to provide a device which issimple in nature and structure and which modulates a large amount ofpower output with good quality modulation by means of a signal ofcomparatively small power.

A further object of the present invention is to provide a novelschemeand means for impressing on high frequency oscillations phasevariations truly representative of the signal so that a good qualitymodulation is obtained and to accomplish the same by the use of aminimum amount of apparatus operating extremely efficiently.

The above objects are obtained in accordance with the present inventionby modulating oscillations of constant frequency at the source ofproduction, that is, in the oscillator. For example, if a crystaloscillator is used to produce constant frequency oscillations,modulation may be accomplished in the crystal oscillator. In a crystaloscillator the crystal voltage Works into the grid to cathode capacityand a resistive component caused by the grid to cathode current of thetubes. This is the input impedance of the tubes and, as the grid currentis caused to vary by changing the applied grid voltage, that is, bychanging the potential applied to the grids of the oscillators at signalfrequency, the grid to cathode resistive components varies and thevariations produce a phase shift proportional to the variations of thegrid to cathode resistances.

Any tube that draws sufficient grid to cathode current, so that theresistive component is sufcient, may be modulated in this manner,whether the tube be an oscillator or an amplier.

The novel features of my invention have been pointed out withparticularity in the claims appended hereto.

My novel method of producing phase variations in oscillations which arecharacteristic of signal currents, and devices for accomplishing thesame will be described in detail hereinafter. In this descriptionreference will be made to the drawings, throughout which like symbolsindicate like parts, and in which:

Figure 1 shows an arrangement for producing oscillations and formodulating the same as to phase in accordance with my novel scheme;

AFigure 2 shows diagrammatically the manner 55 in which phase modulationis accomplished in accordance with the present invention.

Figure 3 is a vector diagram. illustrating the manner in which the phaseshift is obtained in certain voltages in the circuit of Figure 2; while,

Figures 4 to 9 inclusive show various modifications of the arrangementof Figure 1.

A specific embodiment of the invention will now be described. Indescribing said embodiment reference will be made to Figure 1 of theinvention in which, for purposes of illustration,

an oscillator and phase modulating means, ar-

ranged in accordance with the present invention have been shown.

In Figure 1 a pair of thermionic tubes C have their anode electrodes 6and 8 connected, as shown, to a parallel tuned circuit I0. 'Ihe circuitI is tuned to a frequency slightly above the frequency it is desired toproduce so that the circuit I6 as to said frequency is inductive. Thecontrolgrids I2 and I 4 of tubes Care connected together by way of aleak resistance 29, as shown. A piezo-electric crystal PC is connectedas shown between the control grids I2 and I 4 of tubes C. Due to thecapacity between the electrodes 6 and I2 and 8 and I4 and the impedanceof the tuned circuit Ill, oscillations of constant frequency areproduced in thetubes C and their associated circuits when the electrodesof tubes C are energized by direct current potential sources V(notshown) vthe positive terminal of which is connected to the lead markedB| and the negative terminal of which `isV connected to ground or' thecathode return circuit. The-frequency of the oscillations produced isdetermined by the value of the elements included in the circuitsdescribed and in particularby the physical dimensions of thepiezo-electric crystal PC. The anode to cathode and grid to cathodealternating current circuits of Vboth tubes C are completed byconnecting the cathodes to ground by way of radio frequency by-passcondenser C1 connected as shown.

'Ihe cathode-electrodes K are heated Yby means of a lamentary heatingelement and heating circuit 26 which may be energized from any sourcenot shown directly or by Way of a transformer 2l. To prevent highfrequency oscillations from reacting on this transformer each side ofthe filament heating circuit 26 is connected to ground byway of ably-pass condenser Ci as shown.

In operation the circuit I is tuned to a frequency slightly above thenatural frequency of the crystal PC. The external impedance of the platecircuit is high. The resistance 20 furnishes the effective grid bias forthe oscillators due to grid rectification. 'I'he positive potential Amaybe applied to theanode electrodes and 8 by way ofthe tuned circuit I0,which may lbe .connected with any source of direct current potential.

The arrangement just described will, as is Well known, produce sustainedoscillation of good .amplitude and constant frequency, which frequencywill be determined in part by the piezoelectric crystal PC and in partby the tuned cir cuit I0. The oscillations so produced -may be inducedby way of the inductance in circuit I0 on an inductance 22 connected byway of additional Aapparatus 30 to utilization circuit 32.

' The manner in which the oscillations produced in the ltubes C aremodulatedin phase at signal frequency will now be described. A source ofmodulating potentials A, as, for example, an alternating current sourceof constant andaudi- Yproduced in the tubes C to return to the cathodesK by wayv of the by-pass condenser C1 connected as shown so that they donot react by way of the 'transformer 24 on source A. The condenser C1forms a low impedance path by which the radio frequency current mayreturn to the cathode without pasing through the modulator but is ofhigh impedance to the modulating potentials. Thepotentials supplied atsignal frequency to the cathodes K vary the potential of the cathodewith respecttothe grid'electrodes I2 and I4 and thereby'vary theimpedances between the grids and cathodes of said tubes, andconsequently vary thegrid to cathode currents.

VSincethe crystal-voltage is Working into the grid 'to cathodecapacities of the tubes, and the resistive components caused by therespective grid to cathode direct currents, these variations ofpotentials and ofY currents result in a phase shift which is'proportional to the variation of the resistive component in the grid tocathode circuits. This in turn phase modulates the resultant current inthe common anodecircuit of the oscillators C so that high frequencyoscillations,modulated in phase in accordance with the potentials in Aappear in the circuit It and may `be impressed therefrom to a loadcircuit.

The cathodes of tubes C`V are swinging as to `potential inaccordancewith the modulating potentials from source A. The modulatingpotentials are applied in phase to the cathodes so that bothcathodes'swing'in the same direction simultaneously. Assuming that thecathodes both swing positive irrespective of the instantaneous gridradio frequency potential there will be a phase vshiftin the radiofrequency output of each tube. 'I'his phase shift will be in the samedirection. 'I'his phase shift in the same direction of the-radiofrequency energy in the outputs of both tubes results fromthe fact thatthe modulating potentials are in phase on the grids and the carrier wavepotentials are in phase opposition.

The oscillationsso produced and modulated may be passed through anamplitude limiter or a frequency multiplier or an amplifier or one ormore of said several devices, all of which may be included in the unit3Q. The phase modulated amplitude limited or frequency multiplied oramplified oscillations, or both frequency multiplied and amplifiedoscillations, may be Vutilized in any manner. For example, they may beimpressed on a line for transmission or may be impressed, as shown, onVan aerial system 32 for radiation. Furthermoraas indicated above thephase modulated oscillations maybe further modulated in any knownmanner, before transmission. This modulation may take place any place inthe circuits but preferably takes place after the phase modulation asdescribed above is accomplished.

Although it is thought that the manner in which phase modulation hasbeen accomplished will be understood from the above, a further briefexplanation of what takes place in the circuits associated with thetubes C to produce phase modulation at signal frequency will be given.

Cil

In giving this explanation reference will be made to rthe fundamentalFigure 2, which represents the voltages in the circuit and on theelectrodes of either of the tubes C of the arrangement of Figure 1. Tosimplify this explanation it will be assumed that only one tube C isbeing utilized, that is, that only half of the oscillator in Figure l isbeing utilized. 'I'he explanation applies equally to the case Where bothtubes are considered and is made with reference to one tube only forpurposes of simplicity. In explaining this circuit reference will bemade to the vector diagrams of Figure 3 in which:

IR is the grid current;

IRI is the grid current when the resistance R has been changed due tomodulating potentials;

Z is the impedance of the Circuit, the electrodes, etc.;

er is the input voltage;

ez is the voltage on the tube side of the impedance Z;

Iz is the total current in the circuit; and,

Ic is the current through the tube capacity of the circuit.

Referring to Figure 2, G represents the quartz crystal input to one ofthe tubes C. The crystal may be considered as a constant voltagegenerator. We can make such an assumption for the crystal has so muchstored energy (circulating current) that there is a persistency ofoscillation greater than the audio frequency. If the crystal responsecurve (resonance curve) is about iifty cycles Wide then any audiofrequency above that will not affect the output voltage and will notreact back on the crystals frequency cf oscillation. This crystal outputvoltage is impressed into an impedance, made up of series elements andshunt elements. Z represents the series element and R and C the shuntelements. By varying the grid voltages (raising and lowering the cathodevoltage) the resistive element of the shunt impedance is varied. Thegrid current changes and this changes the phase of the grid voltage inaccordance with the input tone voltage ET.

Referring to vector diagram 3, it may be seen that as we change theinput impedance of one of the tubes by Varying R, the phase of the gridvoltage is changed. The oscillations repeated in this tube will belinearly modulated in phase and will appear on the anode of said tubeand in the circuit I0. The impedance input of the other tube C will bechanged at the same time by the change in R and this tube will in likemanner feed oscillating energy linearly modulated in phase but indifferent sense to the circuit I0. The added energy will be of a phasewhich is a resultant of the phases of the separate energies. With thissystem I can get practically linear phase displacement for input tonevoltage over a large enough range so that the effective phase swingafter multiplication in the doubler stages gives a displacement equal to100% in amplitude modulation.

Obviously many different types of crystal control oscillators may beused to replace the oscillator shown in Figure l. Practically any typeof piezo-electric controlled oscillator circuit may be used to producethe oscillations which may in turn be modulated in phase in accordancewith my novel scheme as illustrated in Figure l.

In the arrangement of Figure 4 the oscillator may be of the pushpulltype in which the feed back necessary to insure the production ofconstant oscillations may be obtained by interconnecting the anodes andcontrol grids by way of a piezo-electric crystal PC on which fourelectrodes bear as shown. The anode circuit Hl` may be tuned to afrequency slightly above the frequency of the oscillations generated asdetermined by the crystal PC or if desired may bertuned to the frequencyof said oscillations. The plate to grid electrode of the tube C may beneutralized by capacities NC as shown in Figure 5 or may beunneutralized as shown in Figure 4 depending upon the requirements ofthe particular case and on the degree of stability necessary in theoscillator. The modulating potentials are applied in both these circuitsin a manner similar to which they are applied to the oscillator ofFigure 1, and since phase modulation is accomplished in the circuits ofFigures 4 and 5 in substantially the same manner in which it isaccomplished in the circuit of Figure l, a description of the manner inwhich said phase modulation is accomplished is thought unnecessary atthis point.

In some cases inductive feed back may be desirable. The oscillatorgenerator in this case may be as illustrated in Figure 6, in whichtheanodes 6 and 8 of tubes C are connected together by way of a winding W,which is coupled to a Winding W1, connected to a pair of the electrodesof the piezo-electric crystal system PC as shown. Energy from the anodesof the tubes is fed from the Winding W to the Winding Wr and from thesaid last named winding by Way of the crystal to the electrodes l2 andI4. The winding W1 may be tuned toV the desired frequency, that is, tothe natural frequency of the crystal PC or slightly above said frequencyor to a harmonic thereof by the capacity Cg. To insure magnetic couplingonly between the anodes 5 and 8 and the control grids I4 and l2respectively, an electrostatic shield S may be interposed between thewindings W and W1 as shown. A further isolation of the electrodes, toand from which energy is supplied, may be insured by interposing asecond electrostatic shield S1 between the. respective electrodes of thecrystal PC as shown. If desired, the anodes and control grids of theopposed tubes may be interconnected by neutralizing condensers NC asshown.

The modulating potentials are applied between the cathode and controlgrid of the tubes C to accomplish phase modulation of the oscillationsproduced in said tubes in the same manner in which phase modulation isproduced in the prior modications. 'Ihe load circuit which may beadditional amplifiers or an antenna or a transmission line may becoupled by way of a pair of coupling condensers to the inductance W asshown.

The tubes C of the oscillation generator may be triodes as shown inFigure 6 or may be of the screen grid type as shown in Figure '7. Whentubes of the screen grid type are utilized the screen grid electrodesmay be energized at a positive potential by way of a resistance 34connected as shown through a radio frequency choke RFC to a source ofpotential. The arrangement of Figure 'l may in other respects be similarto the arrangement of Figure 6.

Of course my invention contemplates the use of any type of tubes inthese oscillation generators. For example the triodes C of Figure 4 maybe replaced by screen grid electrode tubes C as shown in Figure 8.

While I have shown the load circuit as being either `inductively -orcapacitively coupled to the output electrodes of the oscillator, it willbe understood that other forms of coupling may be utilized. For example,I may include a resistance 32 in the anode'load circuit and points onthis resistance may be coupled by coupling condensers as shown in Figure9 to the unit 30. Moreover, if desired, the grid to plate capacity maybe balanced or tuned out by connecting variable inductances 36 and 38between the respective control grids and ground as shown in Figure 9.VThe circuit arrangement of Figure 9 is otherwise sufficiently similar tothe circuits described hereinbefore to make unnecessary a detaileddescription of the operation thereof at this point.

Of course, it should be understood that the modulating potentials may beapplied by other circuits than those shown in the prior figures and maybe applied between electrodes of the tubes other than the electrodes towhich they are applied in said circuits. For example, the modulatingpotentials may be fed to the grid or cathode by many diiferent circuitarrangements. Moreover, in a multi-electrode tube the electrodes otherthan the grid and cathode may be likewise modulated without departingfrom the spirit of the present invention.

The modulating potentials in the source A may represent signals which itis desired to convey to a distant point or may be merely for the purposeof wobbling the carrier wave to obtain the benefits of phase orfrequency diversity. In the latter case the wobbled oscillations may bemodulated at signal frequency as to phase Vor frequency or amplitude orany combination of the said types of modulation. Such modulation maytake place in any stage oi the transmitter but preferably takes place ina stage following the phase modulator stage including the generatortubes C.

Having thus described my invention and the operation thereof, what Iclaim is:

1. The combination with an oscillation Ygenerator including a pair ofthermionic tubes, each having an anode, a control electrode, and acathode and a tuned output circuit connected between said anodes, aninput circuit connected with the control electrodes and to ground andfrequency determining apparatus connected with said input circuit, ofmeans for modulating the phase of the oscillations produced comprisingan impedance connecting the cathodes of said tubes to ground, and acircuit for applying modulating potentials to said impedance and fromsaid impedance in phase to the cathodes of said tubes.

2. The combination with an oscillation generator including a pair ofthermionic tubes each having an anode, a cathode, and a controlelectrode, an output circuit tuned to a frequency slightly above thefrequency or the oscillations to be produced connected withthe anodes ofsaid tubes, a, frequency determining element in the input circuitbetween the control electrodes of said tubes, resistive means connectingthe control electrodes to ground, of means for modulating the phase ofthe oscillations produced comprising an impedance connecting thecathodes of said tubes to ground, and a circuit for applying controllingpotentials to said impedance and from said impedancey in phase to thecathodes of said tubes.

3, In combination with an oscillation generator including a pair ofthermionic tubes each having an anode, a cathode, and a controlelectrode, a reactive output circuit connected between the anodes ofsaid tubes, a frequency determining piezo electric crystal in an inputcircuit connected between the control electrodes of said tubes and toground, of means for modulating the phase of the oscillation producedcomprising a transformer having its secondary Winding connected betweenthe cathodes of said tubes and ground and its primary winding connectedto a source of modulating potential.

4. The combination of an oscillator comprising a pair of thermionictubes each having a cathode, a control electrode and an anode, andhaving frequency determining and generating input and output circuitscomprising a reactance connected with the anodes of said tubes, andfrequency determining apparatus connected between the control electrodesof said tubes, and circuits raising the cathodes of said tubes abovegro-und alternating current potential, of means for applmng modulatingpotentials in phase to the cathodes of said tubes to vary the potentialof said cathodes in phase at the frequency of the modulating potentialsto thereby modulate the phase of the oscillations produced, and a loadcircuit coupled to the output circuit of said oscillator.

5. Signalling means comprising a pair of thermionic tubes having theiranodes connected together by a circuit including an inductance, and Vafrequency determining circuit including an inductance coupled to saidfirst named inductance connected between their control-grid electrodes,circuits including an impedance between the control grid electrodes ofsaid tubes and ground vand a source of direct current potential betweenthe anodes of said tubes and ground for energizing the electrodes ofsaid tubes to produce sustained oscillations in said tubes and saidcircuits, and means for modulating the phase of the oscillationsproduced, comprising a circuit including an impedance connected betweenthe cathodes of said tubes and ground, means for applying modulatingpotentials to said impedance and from said impedance to the cathode Yelectrode of each of said tubes, the modulating potentials applied tothecathodes being in phase.

6. Signalling means comprising a pair of thermionic tubes each having ananode, a cathode and a control grid, said tubes having their anodesconnected together by an inductive reactance and a frequency determiningcircuit connected between their control grids, an impedance connectedbetween the control grids of said tubes and the cathodes of said tubes,means for energizing the electrodes of said 'tubes to produce sustainedoscillations in said tubes and said circuits, and means for modulatingthe phase of the oscillations produced, lcomprising a trans.- formerhaving a primary winding coupled to a source of modulating potentials,and a secondary winding connected between the cathode of each of saidtubes and ground.

7. Signalling means comprising a pair of thermionic tubes; each havingan anode, a cathode, and a control grid and having their anodesconnected togetherI by a resonant circuit tuned to substantially thefrequency of the oscillations it is desired to generate, and a frequencydetermining circuit connected between their control grids, means forenergizing the electrodes of said tubes to produce sustainedoscillations in said tubes and said circuits, and means for modulatingthe phase of the'oscillations produced, comprising a resistanceconnected between the control grids and cathodes of said tubes, atransformer having a primary winding coupled to a source o-f modulatingpotential, and a secondary winding connected between the cathodes ofsaid tubes and ground to apply modulating potentials in phase to thecathodes of said tubes.

8. Signalling means comprising a pair of thermionic tubes having theiranodes connected in pushpull relation, a piezo-electric crystal havingpairs of electrodes, a circuit connecting a pair of said crystalelectrodes to the control grid electrodes of said tubes, a circuitconnecting another pair of said crystal electrodes to the anodes of saidtubes, means for energizing the electrodes of said tubes to producesustained oscillations in said tubes and said circuits, and means formodu- J lating the phase of the oscillations produced,

comprising a reactance connecting the cathodes of said tubes to ground,a resistance connecting the control grid electrodes of said tubes toground, and a source of modulating potentials coupled to said reactance.

9. The combination with an oscillator of the pushpull type comprising apair of therrnionic tubes having their anode electrodes coupled inpush-pull relation by way of an impedance and their control gridscoupled in push-pull relation by way of a piezo-electric crystal andtheir cathodes and control grids connected to ground by way ofimpedances, and circuits for energizing the electrodes of said tubes toproduce oscillations therein of a frequency determined by the physicaldimensions of said crystal and means for modulating the oscillations soproduced in phase at signal frequency comprising a circuit for applyingmodulating potentials to the impedance between said cathodes and groundto swing said cathodes relative to ground potential at a modulationfrequency rate.

10. A signalling system including means for producing high frequencyoscillations of constant frequency comprising a pair of thermionic tubeshaving anode, cathode and control grid electrodes, a resistanceconnected between the anode electrodes of said tubes and to a source ofpotential, a resistance connected between the control grid electrodes ofsaid tubes and to ground, a piezo-electric crystal having a pair ofelectrodes connected to the control grids of said tubes, said crystalhaving another pair of electrodes connected to the anodes of said tubes,inductive means for tuning out the capacitive coupling between the gridand plate of each tube, an impedance connected between the cathodeelectrodes of said tubes and ground, said impedance being shunted by aradio frequency bypassing condenser, a source of modulating potentials,and a circuit for applying modulating potentials from said source tosaid impedance.

1l. Means for producing high frequency oscillations of constantfrequency comprising a pair of thermionic tubes having anode, cathodeand control grid electrodes, an impedance connected between the anodeelectrodes of said tubes and to a source of potential, a resistanceconnected between the control grid electrodes of said tubes and toground, a piezo-electric crystal having different electrodes connectedto the control grids of said tubes, said crystal having other electrodesconnected to the anodes of said tubes, a neutralizing condenserconnected between the anode of one of said tubes and the control grid ofthe other of said tubes, a neutralizing condenser connected between theanode of said last named tube and the control grid of said first namedtube, an impedance connected between the cathodes of said tubes andground, said impedance being shunted by a radio frequency bypassingcondenser, a source of modulating potentials and a circuit for applyingmodulating potentials from said source to said impedance.

l2. Means for producing high frequency oscillations of constantfrequency comprising a pair of thermionic tubes each having anode,cathode and control grid electrodes, an inductive reactance connectedbetween the anode electrodes of said tubes and to a source of potential,a resistance connected between the control grid electrodes of said tubesand to ground, a piezo-electric crystal having different electrodesconnected to the control grids of said tubes, said crystal having otherelectrodes coupled to the anodes of said tubes, a neutralizing condenserconnected between the anode of one of said tubes and the control grid ofthe other of said tubes, a neutralizing condenser connected between theanode of said last named tube and the control grid of said one of saidtubes, an impedance connected between the cathodeelectrodes of saidtubes and ground, said impedance being shunted by a radio frequencyby-passing condenser, a source of modulating potentials, and a circuitfor applying modulating potentials from said source to said impedance.

13. Means for producing high frequency oscillations of constantfrequency comprising a pair of thermionic tubes each having anode,cathode and control grid electrodes, an inductance connected between theanode electrodes of said tubes and to a source of potential, aresistance connected between the control grid electrodes of said tubesand to ground, a piezo-electric crystal having two pairs of electrodes,an inductance connected to an electrode of each of said pairs, saidinductance being coupled to said first named inductance, a connectionbetween the other electrode of each of said pair of electrodes and thecontrol grid of a dierent one of said tubes, a shield between said pairsof crystal electrodes, an electro-static shield between saidinductances, and means for varying the potential on the cathode of eachof said tubes at signal frequency.

14. The combination with an oscillator of the pushpull type comprising apair of thermionic tubes having their anode electrodes coupled by way ofan inductive reactance and their control grids coupled by way of apiezo-electric crystal and their cathodes connected to ground by way ofan impedance of means for energizing the electrodes of said tubes toproduce oscillations therein of a frequency determined by the physicaldimensions of said crystal and means for modulating the oscillations soproduced in phase at signal frequency comprising a circuit for applyingmodulating potentials to said impedance.

15. The combination with an oscillator comprising a pair of thermionictubes having their anode electrodes coupled in push-pull relation by wayof an impedance and their control grids cou- Vpled in push-pull relationby way of a piezo elecpass condenser in said connection between thecathodes of said tubes and ground and a circuit for applying modulatingpotentials to said inductive reactance.

16. The combination with an oscillator comprising a pair of thermionictubes having their anode electrodes coupled in push-pull relation by wayof an impedance and their control grids coupled in push-pull relation byWay of a resonant circuit and their cathodes connected together and toground; of means for energizing the electrodes of said tubes to producesustained oscillations therein of a frequency determined by the resonantcircuit including, an impedance connecting the control grids of saidtubes to ground, and means for applying direct current potentials to theanodes of said tubes; and means for modulating the oscillations soproduced in phase at signal frequency comprising an impedance connectedbetween the cathodes of said tubes and ground, and a circuit forapplying modulating potentials to saidrimpedance.

17. In a signalling system, a pair of electron discharge tubes eachhaving an anode, a cathode, and a control grid, an alternating currentcircuit coupling the anodes of said tubes to a work circuit, analternating current circuit coupling the control gridsV of said tubestogether and to a @new resonant frequency determining means, animpedance connecting the control grids of said tubes to ground, andmeans for modulating the phase of alternating current owing in at leastone of said circuits in accordance with modulating potentials comprisinga source of modulating potentials connected between the cathodes of saidtubes and ground.

18. In a signalling system, an oscillation generator comprising a pairof electron discharge tubes each having an anode, a cathode, and a.control electrode, means connecting said anodes to a work circuit, meansconnecting said control grids in an oscillatory circuit in which thecirculating energy loss per cycle of wave energy oi the frequency it isdesired to produce is low, a connection between a point on said lastcircuit and a point of relatively fixed potential, and means formodulating the phase of the oscillations, produced at a modulatingpotential rate comprising a source of modulating potentials having oneterminal connected to the cathodes of said tubes and another terminalconnected to said point of relatively fixed potential to vary thepotential of said cathodes at signal frequency relative to said point ofrelatively fixed potential.

HALLAN EUGENE GOLDSTINE.

